An in vitro transcription system which faithfully reflects in vivo transcription has been developed. This research proposal seeks to define at the molecular level the events leading to the formation of a primary nuclear viral RNA transcript and the processing and modification of that transcript into messenger RNA. The experiments first will identify the transcription regions on the viral DNA by locating the promoter and termination sites. This will define the maximum sized viral transcript derived from that region. Using short pulse labeled RNA the primary viral transcript will be identified and the processing of that RNA transcript will be followed in pulse-chase experiments. The processing will be followed by hybridization analysis of size-fractionated in vitro nuclear RNA on polyacrylamide gels containing 98% formamide. In addition, this system adds poly (A) to the viral transcripts. This fact will be utilized to study the mechanism of poly (A) addition and the role of poly (A) in the processing of mRNA.